This invention relates generally to the utilization of lower alkanes and the synthesis of hydrocarbons therefrom and, more specifically, to the oxidative conversion of low molecular weight alkanes, such as methane, to higher molecular weight hydrocarbons.
As the uncertain nature of ready supplies of and access to crude oil has become increasingly apparent, alternative sources of hydrocarbons and fuel have been sought out and explored. The conversion of low molecular weight alkanes (lower alkanes) to higher molecular weight hydrocarbons has received increasing consideration as such low molecular weight alkanes may be generally available from more readily secured and reliable sources. Natural gas, partially as a result of its comparative abundance, has received a large measure of the attention that has focused on sources of low molecular weight alkanes. Large deposits of natural gas, mainly composed of methane, are found in many locations throughout the world. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of coal, tar sands, oil shale and biomass, for example.
Today, much of the readily accessible natural gas generally has a high valued use as a fuel whether in residential, commercial or in industrial applications. Additional natural gas resources, however, are prevalent in many remote portions of the world, such as remote areas of Western Canada, Africa, Australia, U.S.S.R. and Asia. Commonly, natural gas from these remote resources is referred to as "remote natural gas" or, more briefly, "remote gas."
In many such remote regions, the widespread, direct use of the natural gas as a fuel is generally not currently profitable. Further, the relative inaccessibility of gas from such resources is a major obstacle to the more effective and extensive use of remote gas as the transportation of the gas to distant markets wherein the natural gas could find direct use as a fuel is typically economically unattractive.
Of course, while the primary current use of natural gas is as a fuel, natural gas may alternatively be used as a feedstock for chemical manufacture. In fact, natural gas is a primary chemical feedstock for the manufacture of numerous chemicals, such as methanol, ammonia, acetic acid, acetic anhydride, formic acid, and formaldehyde, for example. However, the markets for such chemicals are fairly limited in size. Consequently, methods for converting low molecular weight alkanes, such as those present in remote natural gas, to higher molecular weight hydrocarbons, preferably, to more easily transportable liquid fuels for which the world market is relatively large and/or elastic, are desired and a number of such methods have been proposed or reported.
Conversion of natural gas to liquid products is a promising solution to the problem of more effectively and efficiently utilizing low molecular weight hydrocarbons from remote areas and constitutes a special challenge to the petrochemical and energy industries. The dominant technology currently employed for the utilization of remote natural gas involves conversion of the natural gas to a liquid form via the formation of synthesis gas, i.e., a process intermediary composed of a mixture of hydrogen and carbon monoxide also commonly referred to as "syngas." In syngas processing, methane, the predominant component of natural gas, although typically difficult to activate, is reacted with oxygen or oxygen-containing compounds such as water or carbon dioxide to produce syngas which in turn is then converted to desired products.
Syngas processing, however, is relatively costly as the production of syngas and the subsequent conversion of the syngas are typically very capital intensive processing schemes. Further, while some of the products to which syngas can be converted, such as methanol, mixed alcohols, acetic acid, etc., contain oxygen and are thus logical products for production via syngas processing, hydrocarbon products such as gasoline and diesel fuel typically do not contain oxygen and consequently the production of such materials via syngas processing requires the additional processing step of oxygen removal. The addition and later removal of oxygen when such products are produced via syngas processing ultimately increases production costs.
When hydrocarbon products such as gasoline and diesel fuel are sought, the syngas mixture can be converted to syncrude, such as with Fischer-Tropsch technology, and then upgraded to the desired transportation fuels using typical refining methods. Alternatively, syngas can be converted to liquid oxygenates which can be blended with conventional transportation fuels to form materials such as gasohol, used as alternative fuels or converted to conventional transportation fuels by catalysts such as certain zeolites.
Because syngas processing typically requires high capital investment, with syngas typically being produced in energy intensive ways such as by steam reforming where fuel is burned to supply the heat of reforming, and represents an indirect means of higher hydrocarbon production (i.e., such processing involves the formation and subsequent reaction of the syngas intermediaries), other means for converting lower alkanes directly to higher hydrocarbons have been sought.
Oxidative coupling has been recognized as a promising approach to the problem of conversion of lower alkanes to higher molecular weight hydrocarbons. The mechanism of action of oxidative coupling processing, however, has not been clearly identified or defined and is not clearly understood. In such oxidative coupling processing, a low molecular weight alkane or a mixture containing low molecular weight alkanes, such as methane, is contacted with a solid material referred to by various terms including catalyst, promoter, oxidative synthesizing agent, activator or contact material. In such processing, the methane is contacted with such a "contact material" and, depending on the composition of the contact material, in the presence or absence of free oxygen gas, and is directly converted to ethane, ethylene, higher hydrocarbons and water. Carbon dioxide formation, which is highly favored thermodynamically, is an undesired product, however, as the formation of carbon dioxide results in both oxygen and carbon being consumed without production of the desired higher value C.sub.2 + hydrocarbons.
In most cases of oxidative coupling processing, carbon monoxide and hydrogen are coproduced in addition to desired C.sub.2 + hydrocarbons. If desired, such coproduced hydrogen can be used alone, in part or in its entirety, or supplemented with hydrogen from another source to effect conversion of carbon oxides to produce methane. Such produced methane can, in turn, be recycled. Alternatively, the hydrogen can be used to effect conversion of carbon monoxide to carbon-containing oxygenates such as methanol or mixed alcohols (e.g., a mixture of one or more alcohols such as methanol, ethanol, propanols and butanols) or higher hydrocarbons such as a mixture of paraffins and olefins typically produced in the process commonly known as Fischer-Tropsch synthesis. Alternatively or in addition, such coproduced carbon monoxide and hydrogen can, if desired, be combined with olefins, such as those produced during the oxidative coupling processing, to produce various oxygenates, such as acetone or propanol, for example. As described above, however, the production of materials such as oxygenates from carbon monoxide and hydrogen (i.e., synthesis gas) is not a direct approach for the utilization of natural gas, as such processing still involves the use of the syngas intermediaries.
Many patents describe processes for converting methane to heavier hydrocarbons in the presence of reducible metal oxide catalysts. During such processing, the reducible metal oxide "catalyst" typically is reduced and thus most of these patents require or imply the need for a separate stage to reoxidize the catalyst.
For example, U.S. Pat. No. 4,444,984 discloses a method for synthesizing hydrocarbons wherein methane is contacted with a reducible oxide of tin at an elevated temperature. Such contact results in the tin oxide being reduced. The reduced composition is then oxidized with molecular oxygen to regenerate a reducible oxide of tin.
U.S. Pat. No. 4,495,374 discloses the use of a reducible metal oxide promoted by an alkaline earth metal in such a method of methane conversion. During such processing, the reducible metal oxide of the promoted oxidative synthesizing agent is reduced. The reduced synthesizing agent can then be removed to a separate zone wherein it is contacted with an oxygen-containing gas to regenerate the promoted oxidative synthesizing agent.
Examples of other such patents include: U.S. Pat. No. 4,523,049, which shows a reducible oxide catalyst promoted by an alkali or alkaline earth metal, and requires the presence of oxygen during the oxidative coupling reaction; U.S. Pat. No. 4,656,155, which specifies yttrium in a mixture requiring zirconium and alkali metal; U.S. Pat. No. 4,450,310, which is directed to coupling promoted by alkaline earth metal oxides in the total absence of molecular oxygen; and U.S. Pat. No. 4,482,644, which teaches a barium-containing oxygen-deficient catalyst with a perovskite structure.
Additional patents and publications describe oxidative coupling of methane using alkaline earth metal-containing halide catalysts. These include:
"Oxidative Coupling of Methane with Alkaline Earth Halide Catalysts Supported on Alkaline Earth Oxides," by K. Fujimoto, S. Hashimoto, K. Asami and H. Tominaga, Chemistry Letters, pp. 2157-2160, (1987); "Selective Oxidative Coupling of Methane Over Supported Alkaline Earth Halide Catalyst, " by K. Fujimoto, S. Hashimoto, K. Asami, K. Omata and H. Tominago, presented at the Sep. 1-2, 1988 Bicentennary Catalysis Conference at Sydney, Australia; and "Selective Oxidative Coupling of Methane Over Supported Alkaline Earth Metal Halide Catalyst," Applied Catalysis, 50 (1989), 222-236, K. Fujimoto, S. Hashimoto, K. Asami, K. Omata and H. Tominaga,
which discuss coupling of methane with alkaline earth halide catalysts supported on alkaline earth oxides. Most of the work presented in these papers focus on the halide chloride. The only fluoride-containing materials examined were: NaF/MgO, MgF.sub.2 /MgO and CaF.sub.2 /CaO, with catalyst performance for the tested materials measured at 15 minutes after the start of the reaction. The fluoride catalysts were prepared by fluoriding the surface of calcium and magnesium oxides by treating them with hydrofluoric acid. Enough acid was added in this fashion to produce a 5 wt. %, as metal halide, loading of each compound. In fact, the 1987 paper states that the promoting effect of halide doping was Cl&gt;Br&gt;F. Both the 1987 and 1989 papers state: "It is clear that MgF.sub.2 is a negative catalyst for MgO." They also report that it is not likely that methane is activated by a metal halide that is supported or supplied from the vapor phase.
In these papers, MgCl.sub.2 /CaO was identified as the most effective, of the materials studied, for the oxidative coupling of methane. The papers identify the loss of Cl.sup.- from the material and that deactivation of C.sub.2 formation can be attributed to the loss of halide ion. The researchers added chloride to the feed on recognizing that chloride was being lost from the catalysts while on stream.
Halogen loss from a catalyst or contact material, such as in the form of a halide, particularly in the presence of water as commonly results from oxidative coupling, can result in a very corrosive effluent stream. To permit the safe handling of such corrosive streams, corrosion resistant materials of construction are required. Substituting corrosion resistant materials of construction for typical construction materials almost invariably increases the capital expenditures required for a facility.
An additional concern relative to the use of a catalyst or contact material which experiences the loss of halogen, is possible formation of even trace amounts of undesirable halogenated compounds such as halogenated aromatics, such as chlorinated phenols and chlorinated biphenyls (PCB's), for example. These halogenated compounds are generally undesired as they raise various health concerns.
Accordingly, contact materials should be halogen-free or not lose significant amounts of halogen when the contact material is subjected to oxidative coupling reaction conditions.
As a class of materials, halides tend to have significantly lower melting points, as compared to their oxide counterpart, with fluorides generally tending to have the highest melting points of the halides. For example, barium chloride (BaCl.sub.2) and barium fluoride (BaF.sub.2) have melting points of 963.degree. C. and 1355.degree. C., respectively, while barium oxide (BaO) has a melting point of 1918.degree. C. Typically, processes for the oxidative coupling of lower alkanes operate at relatively high temperatures (e.g., 750.degree. C. to 900.degree. C.) and yet the contact material must remain hard to maintain the crystal integrity of the material in the reactor. The presence of a low melting point contact material or contact material component can result in the loss of performance by the contact material due to the contact material losing surface area or desired or needed components through volatilization. For fluidizable contact materials, the presence of a molten or "soft" component or material can result in the small fluidizable particles adhering to one another upon passing to cooler regions of the reactor or the process. Masses of such multiple particle, adhered materials are generally not suited for use in fluid bed operations as such masses will tend to sink to the bottom of the reactor vessel.
Thus, there is a need that the contact material exhibit and maintain physical integrity when subjected to oxidative coupling reaction conditions.